This application describes a research program aimed at defining the mechanisms by which vitamin A is delivered to the mammary gland during lactation. It is known that vitamin A is transported in plasma in two well-defined lipid protein complexes: newly absorbed dietary retinol is transported in the lipid core of the chylomicron as retinyl esters, while vitamin A released from liver storage is transported by serum retinol-binding protein as retinol. The specific aims are: 1) to identify the roles of thoese two lipid-protein complexes in transporting vitamin A into the mammary gland and 2) to use the technique of autoradiography to identify the cell type(s) that recognize RBP and/or chylomicron and that transport vitamin A across this epithelial tissue. A combination of biochemical and morphological techniques are proposed. Chylomicrons labeled with (3H)retinyl esters will be employed to monitor transport of core lipids into the lactating mammary gland, liver and other tissues of the intact rat. Using the isolated perfused mammary gland, the uptake of (3H)retinol from RBP and (3H)retinyl esters from chylomicrons will be investigated. Light microscopic autoradiography will be used to localize the interaction of labeled RBP, or chylomicron remnants, with specific types of cells. A third specific aim asks whether the secretion of RBP and retinol from the isolated perfused rat liver is increased during lactation, and thus addresses the question of how peripheral utilization of this nutrient might affect its output from liver stores. This research plan addresses the transport of vitamin A between organs and blood, and is one part of a longer-range objective to understand the delivery of vitamin A to target organs, the intracellular metabolism of vitamin A in the breast, and regulation of the secretion of this nutrient into milk. These studies relate to the physiology of vitamin A transport, and the regulation of vitamin A nutrition during lactation.